| The advantage of photochemical reaction is that the reaction process is clean and highly selective.Common photochemical reactions include photoaddition,photodissociation,photooxidation,and photoisomerization.The photoisomerization reaction of double bonds,in particular,is widely known for its vital role in human vision,vitamin production,and artificial molecular motors.And great interest has been stimulated in conducting research of photoisomerization process both in real life and many scientific research fields.Actually,photoisomerization undergoes a series of complex structural changes of the compound in the excited state,and many excited state photochemical problems are involved in this process,such as light-induced closure ring and photochromic.On the basis of this,this thesis uses the model systems of two types of organic conjugated molecules,namely ?-ionone and azotriazole,to study the excited state photochemical process and clarify the photoisomerization reaction mechanism,with a view to provide theoretical guidance for designing and synthesizing new C=C and N=N double bond photo-switching molecules.In order to conduct in-depth study of the excited state process of these model molecules,a variety of calculation methods are applied in this thesis.The first part uses the fully activated spatial self-consistent field method(CASSCF)and the multi-configuration secondary perturbation theory(MS-CASPT2)to study the photo-induced ring-closure reaction mechanism of?-ionone.While the second part uses CASSCF and MS-CASPT2 to study the photochromic reaction mechanism of 1,1'-azo-1,2,3-triazole,providing theoretical basis for understanding the photochemical process of high-nitrogen compounds as well as for designing new high-nitrogen molecules.This article mainly includes the following two parts:In the first part,the mechanism of ?-ionone's light-induced ring-closure process was studied by adopting the methods of CASSCF and MS-CASPT2 respectively.The results show that during the first step,the dihedral angle a destroyed the conjugation of C1=C2-C3-C4,a structure similar to 1,3-butadiene,in the ionone molecule,consequently,the imbalance between the chain and the ring produces a torsion force,which causes the sudden change of the ? angle to relax such force so that the molecule achieves a balanced geometric structure favorable for conjugation.Due to the change in conjugation,an energy barrier of 8.7 kcal/mol is produced on the potential energy curve of the S1 state,making it difficult for the E-?-ionone(trans)*molecule to reach the step1-S1-min point,therefore,only intersystem crossing can occur from S1 state to T1 state.And it returns from the T1 state to the closed-shell ground state E-?-ionone(cis)structure when the structure relaxes to the step1-T1-min point.In the second thermal process,the energy barrier is quite low that only isomerization process of 2.6 kcal/mol can easily occur.The ground state energy of the E-?-ionone(cis)molecule is 2.1 kcal/mol higher than that of the Z-?-ionone molecule,which indicates that the Z-?-ionone molecule is easier to be detected by experiment than the E-?-ionone(cis)molecule in theory.In the third step,the Z-?-ionone molecule absorbs light energy and is excited to reach the FC point,and the structure of the Z-?-ionone starts to relax from this point as the C-O bond length decreases.With the structure relaxation potential energy curve decreasing,the energy difference between the FC point and the step3-S1-min point reaches 33.5 kcal/mol,generating driving force conducive to the structure of the Z-?-ionone molecule.And when the Z-?-ionone molecule relaxes to the step3-S1-min point,it relaxes without radiation to the closed-shell ground state through step3-S1/S0-CI.As the C-O bond length of step3-S1/S0-CI is 2.3 ?,and that of the highest point of the S0 state potential energy curve is 2.0 ?,it undergoes step3-S1/S0-CI non-radiative relaxation to the closed shell layer,yielding two possible structures for the ground state.This is consistent with the two structures of ?-ionone(Z)and ?-pyran that can be detected after the solution of ?-ionone(E)-trans illuminated in the experiment.To summarize,the photocyclization reaction of ?-ionone compounds comprises three steps.The first step is a light step with the participation of a triplet state,the second step is a thermal step close to no barrier,and the third step is a light step with a larger drive.The isomerization mechanism of the entire light-induced ring closure process is E-?-ionone(trans)?E-?-ionone(trans)*?step1-T1-min?E-?ionone(cis)?Z-?-ionone?Z-?-ionone*?step3-S1-min?step3-S1/S0-CI??-pyran.In the second part,the photochromic mechanism of 1,1'-azo-1,2,3-triazole was studied by applying the CASSCF and MS-CASPT2 methods.The results reveal that in the ground state,the energy of AT(cis)molecule is 8.9 kcal/mol higher than that of the AT(trans)molecule,thus AT(cis)molecule is easily isomerized into AT(trans)molecule under the action of heat,which means the AT(cis)compound should be stored at a low temperature experimentally.The photochromic mechanism of 1,1'-azo-1,2,3-triazole molecules involves the S2 state,and the coupling between the ground state and the excited state is weak,indicating that the electron withdrawing property of the nitrogen chain has an impact on the 1,1'-Azo-1,2,3-triazole photoisomerization reaction,which has a reaction channel of S0-AT(trans)?FC-AT(trans)?S2?S1?S1/S0-CI?S0-AT(cis).Combining the factors of structure and energy,it can be seen that the structure of the stable point S1-min and the conical intersection point S1/S0-CI differ greatly,as a result,the non-adiabatic relaxation process cannot be easily achieved.This is mere the beginning of studying the mechanism of high-nitrogen compounds.In the future,future research will focus on deepening understanding of the influence of different positions and numbers of N units on the photochemical process,as well as on designing new high-nitrogen compound molecules in combination with the substituent effect.It is hoped that new types of light-enhanced molecule containing N=N double bond can be designed. |
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